1. Field of the Invention
The invention relates to molecular imaging, more particularly to mass spectrometric imaging under ambient conditions using electrospray-assisted laser desorption ionization mass spectrometry.
2. Description of the Related Art
Imaging mass spectrometry (IMS) is widely used in the investigation of chemical or molecular distributions of solid samples, such as metals, polymers, semiconductors, and geological substances.
Many attempts have been made to explore the feasibility of using imaging mass spectrometry in studying spatial distribution of proteins in various organs. However, due to the biological nature of target protein, e.g., being more labile to ionization energy and being in a state of flux, such efforts did not prove to be satisfactory.
One of the currently-used methods of imaging mass spectrometry is the secondary ion mass spectrometry (SIMS). However, SIMS is only capable of detecting analytes such as metal ions or small organic molecules, and is unable to detect macromolecules such as peptide or proteins because the macromolecules are either spoiled during ionization or unable to be effectively desorbed from the surface of the sample.
Another currently-used imaging method is the method of matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). Although MALDI-MS is capable of successfully desorbing peptide or protein molecules from a solid biological sample, and the result thereof is used to distinguish abnormal or cancerous tissues from normal tissues, several drawbacks still exist for MALDI, such as involving a tedious preparation work and requiring to be conducted in vacuum, etc.
Yet another currently-used imaging method is the method of desorption electrospray ionization mass spectrometry (DESI-MS), which is capable of studying a variety of compounds falling within a wide range of molecular weights, and which is capable of performing direct protein mass spectrometric analysis on a freely moving tissue slice. However, there are several disadvantages involved in DESI-MS, including the difficulty in controlling the precision of striking electron-carrying spray droplets onto the tissue slice, and the inability in desorbing protein molecules from the tissue slice.
It can be seen from the above that a variety of difficulties and inconveniences are encountered when obtaining molecular images through the methods of mass spectrometry. Since spatial analytic information of proteins in organs or tissues is extremely important in medical and biotechnological fields, there exists a need for a mass spectrometric imaging method that is capable of conducting rapid, convenient, and accurate spatial analysis on solid biological samples.